Flowmeters to determine the amount of flowing medium using an orifice and a resistor in a cavity having inlet and outlet openings are known from Offenlegungsschrift DE 37 13 542. A flow velocity measuring device can be inferred from this document. Owing to the pressure differential that is measured upstream and downstream of an orifice, the flow velocity of the medium is determined. The opening surface of this orifice is variable, so that the differential pressure can be held constant by means of an actuator as a function of the flowing medium. This orifice is firmly integrated into the body of the flow velocity measuring device. The tapered valve body is pressurized with a spring force in the direction of the opening of the orifice, so that the actuator opens against the spring force of the flow opening of the orifice. The differential pressure is measured in this device by means of two sensors, which produce an electrical signal and convey that signal to an electronic circuit. From this differential pressure and the lift of the valve body, a flow coefficient is calculated in order to determine the flow velocity.
The drawback of this prior art device is that the mechanism is too complicated and, therefore, its manufacture is not cost effective.
Furthermore, with such a device the flow volume is limited to a relatively small flow rate per unit of time and the reaction surface on the orifice cannot be made arbitrarily large during its construction, so that definite limitations must be accepted.
When braking or releasing a pneumatic railway brake, rates of air flow per unit of time may range from a few liters per minute up to &gt;6,000 liters per minute in extreme cases. The requisite amount of air depends on the length of the air pressure line, i.e., for the application of a pneumatic brake of a train, the length of the train. Since this length varies widely, the rate of air flow fluctuates widely.
When a new train is assembled, one proceeds on the assumption of full brake application, and thus the volume of supplied air is measured up to the full release of the train and this value is stored. At every additional release operation, the volume flowing into the main line during the release operation is always compared with the stored value, whereby changes in the train configuration (e.g., inadvertent closing of a shutoff valve in a car) or other abnormal occurrences can be detected.
Therefore, it is important to determine relatively accurately the flowing gas during brake control. Such flow measurement can be made by the so-called free float flow measurement method, which is based on a resistor body around which a medium flows. The resistor body is either located in a conical measurement pipe or the resistor body itself is conical and located near the flow opening of an orifice meter. The resistance value c.sub.w of the free float body varies as a function of the distance to the orifice meter. Thus, the force F exerted by the medium on the resistor body is proportional to the resistance value c.sub.w. The correlation of the force F with the flow velocity results from the law of resistance known from aerodynamics.